Asteraceae

Asteraceae, also called Compositae, the aster, daisy, or composite family of the flowering-plant order Asterales. With more than 1,620 genera and 23,600 species of herbs, shrubs, and trees distributed throughout the world, Asteraceae is one of the largest plant families.

Asteraceae is important primarily for its many garden ornamentals, such as ageratums, asters, chrysanthemums, cosmos, dahlias, marigolds (Tagetes), and zinnias. Other well-known garden plants and wildflowers include Boltonia, Brachycome, burdock (Arctium), butterbur (Petasites), Calendula, cat’s ear (Hypochoeris), cudweed (Filago and Gnaphalium), Gerbera, hawksbeard (Crepis), Inula, Matricaria, and Piqueria. Some genera include noxious weeds, such as dandelion (Taraxacum), ragweed (Ambrosia), and thistle (Carduus, Cirsium, and others). Several other members of Asteraceae have economic importance as food crops. Artichokes (Cynara), lettuce (Lactuca), endive (Cichorium), and salsify (Tragopogon) are commonly eaten as vegetables, and the edible seeds of safflower (Carthamus), and sunflower (Helianthus) are used in the production of cooking oils. Wormwood (Artemisia) is the source of the poisonous oil used to give the liqueur absinthe its distinctive character.

Members of the family have flower heads composed of many small flowers, called florets, that are surrounded by bracts (leaflike structures). Bell-shaped disk florets form the centre of each head. Strap-shaped ray florets extend out like petals from the centre and are sometimes reflexed (bent back). Some species have flowers with only disk or only ray florets. The sepals have been reduced to a ring of hairs, scales, or bristles that is called the pappus on the mature fruit. The one-seeded fruit (an achene) has a hard outer covering.

The leaves of Asteraceae are simple or occasionally compound, and their arrangement along the stem may be opposite, alternate, or, less commonly, whorled; not infrequently they are opposite toward the base of the stem and alternate above.

Paleobotanists suggest that the first members of this family may have evolved in Argentina some 50 million years ago, based on the discovery of well-preserved fossils that date to the Eocene Epoch (56 million to 33.9 million years ago).